FIG. 14 illustrates a most-commonly-used conventional twisted waveguide, which is a rectangular waveguide having a twisted structure. Since a rapid twisting of a twisted waveguide having such a structure is not allowed during its manufacturing process, the waveguide requires a predetermined length in the propagation direction of an electromagnetic wave. Moreover, the waveguide also requires a large space in the joint portions. Japanese Unexamined Patent Application Publication No. 62-23201 (“Patent Document 1 ”) discloses a structure for solving these problems. Specifically, FIG. 15 illustrates the structure of a twisted waveguide according to Patent Document 1. In this twisted waveguide, a second rectangular waveguide element 2 is attached in a manner such that the second rectangular waveguide element 2 is inclined at a predetermined angle with respect to a first rectangular waveguide element 1. Furthermore, a resonant window or filter window 3 having a transmission center frequency as a predetermined frequency is disposed between the first rectangular propagation path element and the second rectangular waveguide element 2 such that a plane of polarization is inclined at ½ of the predetermined angle mentioned above.